Crystalline polyethylene terephthalate (PET) is widely used for production of is various light weight molded or extruded articles due to excellent mechanical properties of the crystalline PET. However, in some applications, amorphous polyesters are desirable over crystalline polyesters due to their enhanced optical clarity, impact resistance and ease of injection molding. Amorphous copolyesters are widely used for packaging and display materials, printing substrates, etc.
Various proposals to modify PET polyester to reduce or eliminate its crytallinity have been disclosed in the art. To slow down the crystallization and eliminate crystallinity the crystalline structure of a polymer needs to be disrupted with an impurity. Comonomers that do not fit within a crystalline unit cell of a crystallizing polymer may serve as such impurity and successfully disrupt the crystalline structure. Disruption of the crystalline structure results in inhibition of crystallization rate and/or a decrease of the degree of crystallinity. However, the amount of the impurity or disruptive comonomer that needs to be incorporated into PET to completely eliminate crystallinity is quite high, which results in substantial alteration of the PET original mechanical and thermal properties.
The most common amorphous polyester based on polyethylene terephthalate is a PET copolymer modified by about 35 mole % of cyclohexanedimethanol (CHDM) comonomers. These polymers are commercially available from, for example, Eastman Chemical Company under various trade names such as PETG. The CHDM used for modification of PET in such copolymers usually consists of approximately 30/70 cis/trans monomers that is routinely obtained by catalytic hydrogenation of dimethyl terephthalate. However, the cost of CHDM monomers is high, which drives up the cost of PETG copolymers.
Japanese Patent Publication JP 05-005026 (published January 1993) discloses polyesters consisting of saturated norbornane monomeric units. While the polyester offers excellent heat resistance and transparency, the polyester disclosed in the reference is expensive.
Japanese Patent Publication JP 09-095820 (published April 1997) discloses high shrinkage fibers comprising copolyesters of ethylene glycol, terephthalic acid and tricyclodecane dimethanol. The copolymers, however, are semicrystalline with high melting enthalpy.
Japanese Patent Publications JP 05-155963 (published June 1993) and JP 05-155964 (published June 1993) disclose low molecular weight copolyester oligomers obtained from aliphatic diols, adipic acid and one or more norbornane dicarboxylic acids. The oligomers are polymerized with polyisocyanate to obtain polyurethanes.
Japanese Patent Publication JP 5-097960 (published April 1993) discloses polyurethanes based on polyols comprising norbornane derived monomers, such as monomeric units derived from norbornane-2,3-dimethanol.
Japanese Patent Publication JP 3-200830 (published September 1991) discloses polyesters and polyester-carbonates comprising a skeleton of norbornane, perhydrodimethanonapthalene and perhydrotrimethanoanthracene.
Japanese Patent Publications JP 8-269856 (published October 1995), JP 10-158935 (published June 1998) and JP 8-209451 (published August 1996) disclose composite fibers comprising a semicrystalline polyester based on PET or PBT and copolymerized with monomeric units having norbornane skeletons and a second polyester.
Japanese Patent Publication JP 07-048443 (published February 1995) discloses shrinkable films comprising polyesters derived from terephthalic acid, ethylene glycol and norbornane diol.
Japanese Patent Publication JP 05-117326 (published May 1993) discloses photocurable oligomers obtained from acrylic acid, 1,6 hexanediol and norbornane-2,3-dicarboxylic acid.
U.S. Pat. Nos. 5,571,620, 5,646,237, and 5,709,940 to George et al disclose sulfocopolyesters obtained by reacting difunctional sulfomonomers with ethylene glycol, polyethylene glycol and a C.sub.4 -C.sub.12 dicarboxylic acid, including 2,5-norbornane dicarboxylic acid.
U.S. Pat. No. 5,567,796 to Nakatsukasa et al discloses polyester fiber obtained from a polyester comprising up to 20% based on dicarboxylic compounds of a norbornane compound. The reference teaches desirability of crystallinity in the polyesters and discloses polyesters comprising monomer units derived from a cis-norbornane-endo-2,3-dicarboxylic compound.
The present invention provides low cost amorphous polyethylene terephthalate based polyesters that contain monomeric units derived from ester forming saturated norbornane compounds. Incorporation of up to about 20 mole % of such units, based upon the total moles of ester-forming components, (or up to about 40 mole % based upon either dicarboxyl or diol residues) results in a polyester that exhibits a desirable, substantially non-crystalline behavior after normal melt-processing to provide an enhanced optical clarity, impact resistance and ease of injection molding.